FIELD OF THE INVENTION
This invention relates to a method for cold-working annular workpieces by applying thereto an internal fluid pressure which causes plastic deformation and enlargement in the dimensions thereof, and more particularly to a method and apparatus for cold-working annular workpieces aimed at uniform working in conformity with the applied inner pressure and at continuous deformation-working.
Mechanical parts which are required to be non-magnetic and to have a high strength, like non-magnetic retainer rings for large generators, have thus far been produced by cold (room temperature)--or warm (350.degree. C.)--working a Mn/Cr-type austenite steel for work hardening. For working, the enlargement in diametric dimensions is easy due to the annular shape and has been put in practice.
In this connection, the conventional methods resort to (1) ring forging or (2) expansion by expanding dies using a combination of segments and a press-in plug. However, these methods invariably require equipment of large scale (in size and pressing power of the equipment and the jigs or tools to be used) and result in irregular mechanical properties due to non-uniform working in the circumferential and axial directions. This is apparently attributable in the case of ring forging to non-uniform deformation (flow) of material caused by frictional contact with the surface of the anvil block and in the case of the segments method to the clearances which are formed between segments at the time of expansion and the non-uniform contact of the outer and inner peripheries of the segments and the annular workpiece caused by the difference in curvature of the inner diameter of the expanded cylindrical material and the outer periphery of the segments and the inner diameter of the annular workpiece before working. Further, upon expanding the annular workpiece, although it additionally undergoes reductions in the axial direction and in its thickness, these additional changes in dimension are restricted by the frictional contact with the tool to thereby increase the residual stress in the axial and radial directions.
The retainer ring for a generator is a body which is rotated at high speed, so that a problem is encountered in that the circumferential residual stress which has a self-balancing power, tending to retain balance three-dimensionally (sum of integration=0), is necessarily increased by increases in axial and radial residual stresses, having an adverse effect on the resulting products. In view of these problems, it is preferable to resort to static working by application of an internal pressure utilizing a liquid as a pressure medium.
Methods for working the non-magnetic retainer rings of generators by liquid pressure are disclosed in French Pat. No. 1,229,861, applicant's Japanese Patent Publication No. 40-8832 and USSR Pat. No. 880,523 (Japanese Patent Publication No. 52-2502), in which the hole of the annular workpiece is closed and sealed by lids of trucated cone shape which are wedged into the openings at the axially opposite ends of the hole. This is disadvantageous for the following reasons. With regard to the equipment used, since the diametric enlargement of the workpiece is accompanied by reductions in its axial dimension, the truncated-conical lids which seal the workpiece have to be moved axially toward each other upon enlarging the workpiece diameter, by means of cylinders. Due to the necessity for cylinders which are large in size and stroke length, the application of high pressure becomes difficult from the standpoint of strength and it is necessary to provide equipment of large scale like a forging press.
Further, when working annular workpieces with small axial dimensions, it is sometimes experienced that the smaller ends of the truncated conical lids which are opposed within the annular workpiece hit against each other during the diametric enlargement of the annular workpiece, loosing the sealing function to make further operation impossible. Further operation is possible only after replacement of the lids, an operation which is troublesome.
With regard to the quality, a deleterious (local) work-hardening layer is formed on the annular workpiece due to imposition of pressure and sliding movement resulting from direct contact with the lids. This is undesirable for the products and the removal of such layers invites a drop in yield. Further, the use of the sealing lids necessarily enlarges the diameter around the marginal edges of the openings at the opposite ends of the annular workpiece by the wedging action of the lids, giving rise to the possibility of causing irregularities in mechanical and physical properties and to a possible increase of residual stress by non-uniform pressurization.